Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
command used to remove objects such as files, directories, device nodes, symbolic link
Symbolic link
In computing, a symbolic link is a special type of file that contains a reference to another file or directory in the form of an absolute or relative path and that affects pathname resolution. Symbolic links were already present by 1978 in mini-computer operating systems from DEC and Data...
s, and so on from the filesystem. To be more precise, rm removes references to objects from the filesystem, where those objects might have had multiple references (for example, a file with two different names), and the objects themselves are discarded only when all references have been removed and no programs still have open handles to the objects.
This allows for scenarios where a program can open a file, immediately remove it from the filesystem, and then use it for temporary space, knowing that the file's space will be reclaimed after the program exits, even if it exits by crashing.
rm generally does not destroy file data, since its purpose is really merely to unlink references, and the filesystem space freed may still contain leftover data from the removed file. This can be a security concern in some cases, and hardened versions sometimes provide for wiping out the data as the last link is being cut, and programs such as shred
Shred (Unix)
shred is a Unix command that can be used to securely delete files and devices so that they can be recovered only with great difficulty with specialised hardware, if at all. It is a part of GNU Core Utilities.-Background:...
are available which specifically provide data wiping capability.
Example
To remove a file named "foo" from a directory one could type:
% rm foo
Normally, no output is produced by rm, since it typically only generates messages in the event of an error. The -v option can be used to get rm to detail successful removal actions.
Users concerned about removing files unexpectedly - particularly when using wildcards - sometimes use the -i option to cause rm to verify each removal in advance, although this method has its own problems.
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
-derived operating systems, which typically do not provide for recovery of deleted files through a mechanism like the recycle bin
Recycle bin (computing)
In computing, the trash is temporary storage for files that have been deleted in a file manager by the user, but not yet permanently erased from the physical media...
., hence the tendency for users to enclose rm in some kind of wrapper to limit accidental file deletion.
There are undelete utilities that will attempt to reconstruct the index and can bring the file back if the parts were not reused.
Options
Common options that rm accepts include:
-r, which removes directories, removing the contents recursively beforehand (so as not to leave files without a directory to reside in) ("recursive")
-i, which asks for every deletion to be confirmed ("interactive")
-f, which ignores non-existent files and overrides any confirmation prompts ("force"), although it will not remove files from a directory if the directory is write protected.
The C shell is a Unix shell that was created by Bill Joy while a graduate student at University of California, Berkeley in the late 1970s. It has been distributed widely, beginning with the 2BSD release of the BSD Unix system that Joy began distributing in 1978...
The Bourne shell, or sh, was the default Unix shell of Unix Version 7 and most Unix-like systems continue to have /bin/sh - which will be the Bourne shell, or a symbolic link or hard link to a compatible shell - even when more modern shells are used by most users.Developed by Stephen Bourne at AT&T...
function of "rm -i" so as to avoid accidental deletion of files. If a user still wishes to delete a large number of files without confirmation, they can manually cancel out the -i argument by adding the -f option (as the option specified later on the expanded command line "rm -i -f" takes precedence). Unfortunately this approach generates dangerous habits towards the use of wildcarding, leading to its own version of accidental removals.
rm -rf (variously, rm -rf /, rm -rf *, and others) is frequently used in jokes and anecdotes about Unix disasters . The rm -rf variant of the command, if run by a superuser
Superuser
On many computer operating systems, the superuser is a special user account used for system administration. Depending on the operating system, the actual name of this account might be: root, administrator or supervisor....
on the root directory, would cause the contents of nearly every writable mounted filesystem on the computer to be deleted, up to the point the system itself crashes from missing some crucial file, directory, or the like.
xargs is a command on Unix and most Unix-like operating systems used to build and execute command lines from standard input. Under the Linux kernel before version 2.6.23, arbitrarily long lists of parameters could not be passed to a command, so xargs breaks the list of arguments into sublists small...
to supply a list of files to delete:
xargs rm < filelist
Or, to remove all PNG images in all directories below the current one:
find . -name '*.png' -print0 | xargs -0 rm
Permissions
Usually, on most filesystems, deleting a file requires write permission on the parent directory (and execute permission, in order to enter the directory in the first place). (Note that, confusingly for beginners, permissions on the file itself are irrelevant. However, GNU rm asks for confirmation if a write-protected file is to be deleted, unless the -f option is used.)
To delete a directory (with rm -r), one must delete all of its contents recursively. This requires that one must have read and write and execute permission to that directory (if it's not empty) and all non-empty subdirectories recursively (if there are any). The read permissions are needed to list the contents of the directory in order to delete them. This sometimes leads to an odd situation where a non-empty directory cannot be deleted because one doesn't have write permission to it and so cannot delete its contents; but if the same directory were empty, one would be able to delete it.
If a file resides in a directory with the sticky bit
Sticky bit
In computing, the sticky bit is an access-right flag that can be assigned to files and directories on Unix systems.-History:The sticky bit was introduced in the Fifth Edition of Unix in 1974 for use with pure executable files. When set, it instructed the operating system to retain the text segment...
set, then deleting the file requires one to be the owner of the file.
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
introduced "rm -rf /" protection in Solaris 10, first released in 2005. Upon executing the command, the system now reports that the removal of / is not allowed. Shortly after, the same functionality was introduced into FreeBSD
FreeBSD
FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...
GNU is a Unix-like computer operating system developed by the GNU project, ultimately aiming to be a "complete Unix-compatible software system"...
rm refuses to execute rm -rf / if the --preserve-root option is given, which has been the default since version 6.4 of GNU Core Utilities
GNU Core Utilities
The GNU Core Utilities or coreutils is a package of GNU software containing many of the basic tools, such as cat, ls, and rm, needed for Unix-like operating systems...
was released in 2006.
User-Proofing
Systems administrators, designers, and even users often attempt to defend themselves against accidentally deleting files by creating an alias or function along the lines of:
alias rm="rm -i"
rm { /bin/rm -i "$@" ; }
Unfortunately, this tends to train users to be careless about the wildcards they hand into their rm commands, as well as encouraging a tendency to alternately pound y and the return key to affirm removes - until just past the one file they needed to keep. Users have even been seen going as far as "yes | rm files".
A compromise that allows users to confirm just once, encourages proper wildcarding, and makes verification of the list easier can be achieved with something like:
if [ -n "$PS1" ] ; then
rm
{
ls -FCsd "$@"
echo 'remove[ny]? ' | tr -d '\012' ; read
if [ "_$REPLY" = "_y" ]; then
/bin/rm -rf "$@"
else
echo '(cancelled)'
fi
}
fi
It's important to note that this function should not be made into a shell script, which would run a risk of it being found ahead of the system rm in the search path, nor should it be allowed in non-interactive shells where it could break batch jobs. Enclosing the definition in the if [ -n "$PS1" ] ; then .... ; fi construct protects against the latter.
There exist third-party wrappers that prevent accidental deletion of important files, like "safe-rm".
In Unix-like operating systems, unlink is a system call and a command line utility to delete files. The program directly interfaces the system call, which removes the file name and directories like rm and rmdir...
In computing, a system call is how a program requests a service from an operating system's kernel. This may include hardware related services , creating and executing new processes, and communicating with integral kernel services...
A conventional computer operating system usually segregates virtual memory into kernel space and user space. Kernel space is strictly reserved for running the kernel, kernel extensions, and most device drivers...
In computing, del is a command in various DOS, OS/2 and Microsoft Windows command line interpreters such as COMMAND.COM, cmd.exe, 4DOS/4NT and Windows PowerShell. It is used to delete one or more files or directories from a filesystem. It is analogous to the Unix rm command...
deltree is a command line command in Microsoft operating systems that deletes an entire subdirectory of files. It appeared in MS-DOS 6, and was retained throughout all Windows versions based upon MS-DOS, including Windows 95, Windows 98, and Windows Me...
External links
The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.
